In recent years the shoe industry, as well as other manufacturing industries using large amounts of leather and/or cloth, have turned to the substitution of thermoplastic materials for uses formerly employing these more expensive materials. Investigations and experimentation have also devised methods of assembly which may be carried out with the use of automatic machinery on these thermoplastic materials, which have proven to be more economical than the hand labor involved in the assembly of leather and/or cloth materials.
Commercial acceptance of thermoplastic materials as substitutes for the more traditional materials has at times required that the thermoplastic material be processed to resemble the more traditional materials such as leather or cloth. In the shoe industry, for example, decorative bits of leather or cloth which are employed in the shoe upper are frequently of a different color than the adjacent material of the upper. Serious attempts have therefore been made to provide for the application of overlay designs to shoe upper parts of PVC, or other plastic material, to enhance the design of the finished product and to duplicate designs which heretofore would be stitched on the leather or cloth material. In providing the embossing and other features such as simulated stitching, etc., the industry has generally gone to the flow molding process in which a die having the desired features is pressed against the workpiece, placed in a mold, and the workpieces subjected to RF heating under pressure to produce the features on the workpiece surface. To provide colored overlays, such as under consideration in the present invention, the overlays are generally cut to final size and placed by an operator in cavities in the silicone flow molding mold, the workpiece placed in the mold, and the welding or combining of the whole area of overlay achieved by heating under pressure during the flow molding process. However, serious problems exist in practicing this technique.
One of the problems that exists in applying separate overlays to the cavities in a silicone flow molding mold is that of the extensive time consumed during the operation in laying into the mold many small colored pieces needed for some designs. This increases the cost of labor, and the process cost, thus increasing the basic cost of the finished product. Additionally, the positioning of color is restricted to cavities which have been formed in the mold. It can therefore be appreciated that if color could be applied to high points in the mold, greater design latitude would exist and a greater freedom of selecting which areas are to be overlay color, versus base material color, would be obtained. The results would be a better use of material both functionally and costwise.
Another problem existing in the aforementioned practice is that in order to position the overlay stock in the mold, there is generally a need to provide thicker overlay stock material to allow for placement ease as the increase mass of material aids in keeping the overlay in place while adding the base piece to the mold. While the initial thicknesses of overlay material in the past were in the range from 0.08 inch to 0.012 inch, this above requirement has increased the average thickness to the 0.018 inch. It will be obvious that this increase of material thickness, while adding to the ease of manipulation by the operator, increases the cost of material and, what is more important, makes the resultant part stiffer. This stiffness results in a very undesirable feature for color application in flex areas of the workpiece. It might further be observed that even with the above described positioning procedure, the positioning is frequently imperfect requiring hand inking after molding to correct errors in placement of the overlay on the workpiece.
It should also be brought out that in conventional tear seal applications, where the heat sealing occurs only at the periphery of the applique there will be air entrapment between the applique and workpiece in the final product. Notwithstanding the application of pressure to the applique surface during the tear seal process, it has been found that the composite of applique and workpiece will invariably develop air bubbles therebetween upon release from the die, or shortly thereafter. This results in a product of generally undesirable appearance.
Another disadvantage to the present practice occurs in shoe overlays which apparently extend over the lasting margin. Such an application would allow the use of conventional tear seal application of the colored overlay which seals the material only at an adjacent narrow band extending around the overlay. Such peripheral bonding and tear seal applications are commonly found in upholstery and handbag manufacture, which are not flow molded after tear sealing. However, a full peripheral tear seal as required for many colored area designs would trap air between the overlay and the workpiece, which appears as a blister in the flow molded parts.